The invention provides an alkali soluble polymer including a specific vinylketone phenol and a derivative thereof as radical polymerizable monomers and a positive working photosensitive resin composition containing the alkali soluble polymer and a photosensitizing agent. According to the invention, there can be provided an alkali soluble resin having high solvent resistance, high water resistance, high acid resistance, high alkali resistance, high thermal resistance, high transparency, excellent adhesiveness with a substrate, and the like and useful for the formation of a patterned resin film obtained by developing in an aqueous alkali solution and a positive working photosensitive resin composition including such an alkali soluble resin.